The present invention relates to a linear potentiometer for measuring travel and in particular the relative linear movement between two parts or components. More particularly, the present invention relates to a linear potentiometer for measuring travel, particularly for controlling the relative movement between two component parts of an injection molding machine, for example a plastic injection molding machine, which potentiometer is of the type including: a metal housing, which is securable to one of the component parts, for an elongated resistance element, with the housing having an elongated tubular shape of approximately rectangular cross section and a longitudinal slit, which is disposed symmetrically with respect to a vertical plane of symmetry, in its upper wall and above the resistance element; a covering band of spring steel which normally rests on the upper wall of the housing to cover the longitudinal slit, and is normally retained there with the aid of permanent magnets on the housing; and a slide which is connectable via a joint to the other component part and which carries a wiper for the resistance element, with the slide having a slide body portion whose lower end carries the wiper and is connected to a pair of horizontal spaced guide ribs slidably supported on guide shoulders formed in the interior of the housing and whose upper end, in its middle section, extends out of the longitudinal slit above the housing, an anchoring plate for the joint connected to the upper end of the slide body portion above the housing with the plate extending horizontally and crosswise to the vertical plane of symmetry, and an arrangement whereby a portion of the covering belt can ride over at least a part of the slide body and around the portion containing the wiper as the slide moves along the housing.
In conventional linear potentiometers of this type (for example, see advertising by Novo-Technik KG, Offterdinger GmbH & Co., Horbstrasse 12, D-7302 Ostifldern 1), the slide structure comprises the slide body, separate guide ribs, an anchoring plate for the joint and a separate, lid-like housing for the joint secured to the anchoring plate, as well as a special guide device for guiding the covering band in the slide. To avoid strains and excessive friction between the plastic guide ribs and their guide tacks, one of the two guide ribs is pivotably supported about a longitudinal axis of the metal slide body. With this kind of structure, the slide represents a relatively large mass, which must be rhythmically braked and accelerated when the linear potentiometer is in operation. Although such linear potentiometers are distinguished by high accuracy in measurement as well as by high wiper speeds, it has nevertheless been found that the precision of measurement drops when the linear potentiometer is subjected to extreme loads over relatively long periods of time. This is particularly the case when the structural components undergo severe vibration and when they are constrained relative movements, deviating from the slide axis, between the housing and the slide. It is also applicable, however, when shock and impacts are frequent, for instance because of improper handling, which can happen especially in the case of exports to industrially underdeveloped countries.
In the conventional linear potentiometers described, the prerequisite for satisfactory protection from dirt of the vulnerable wiper contacts in the housing is to construct the slide from a plurality of parts, some of them plastic and some for them metal. For example, covering of the longitudinal housing slit through which the slide body extends is achieved by providing that a metal bearing part for the slide joint, which part is disposed outside the housing, is screwed onto the anchoring plate of the slide, and that the covering band which covers the longitudinal slit is guided in between the screwed-together components. Moreover, embodying the guide ribs as separate plastic elements attached to the metal slide body, some of them in articulated fashion, is intended to assure strain-free sliding of these guide ribs in the guide shoulders of the housing. In this sense, the multiple-part, multiple-material structure of the known slide is dictated partly by assembly considerations and partly by considerations of function. Such a structure, however, leads to the above-mentioned restrictions in terms of measurement accuracy when there is intensive use over many years, in particular when there is additional strain in the form of shock and impact.
It is also known from French Patent No. 2,142,129, to construct a linear potentiometer such that it is completely dustproof and moistureproof. However, the linear potentiometer disclosed in this patent is not designed to measure the relative movement between two component parts of a processing machine, and accordingly it does not need any connection, effected via a link, between a slide disposed on one component part and the other component part. Thus there is no compelling reason for the slide to have a multiple-part structure. Nevertheless, in this linear potentiometer the portion of the slide located outside the housing and the portion of the slide disposed inside the housing are each embodied by separate molded part. Although these molded parts are firmly joined to one another, they still each have their own guide devices, which are guided at different levels in the housing.